This invention relates to an audio amplifier, and more specifically to an input circuit of an audio amplifier capable of being direct-coupled to a signal source.
Generally, a first-stage amplifier circuit of an audio amplifier is formed of a differential amplifier circuit. When the differential amplifier circuit is composed of bipolar transistors, a coupling capacitor is connected between the base of one transistor and an input terminal which is connected to a signal source such as a phono pickup cartridge, and a bias resistor to supply bias voltage to the base of the transistor is connected between the base of the transistor and circuit ground. Since the input impedance of the differential amplifier circuit depends on the bias resistor, the resistance value of this bias resistor cannot be reduced below a certain value. Therefore, a coupling capacitor is used in order not to supply back to the signal source a DC potential established at the base of the transistor, which DC potential is determined by the resistance value of the bias resistor and the value of the base bias current of the transistor in zero-signal condition. In order to apply signals of fully low frequency from the signal source to the differential amplifier circuit, it is necessary to use a coupling capacitor with a considerably large capacitance. It is known, however, that large-capacity coupling capacitors would deteriorate the quality of audio signals.
If the differential amplifier circuit is formed of field effect transistors (FET's) as have frequently been done of late, any current will hardly flow through a bias resistor which is connected between the gate of an FET and circuit ground, owing to the high impedance property of FET's. As a result, the necessity of using a coupling capacitor is obviated. However, since FET's have a lower mutual conductance Gm than that of bipolar transistors, the signal-to-noise ratio of an amplifier using FET's as first-stage amplifying elements will be lowered as compared with that of an amplifier using bipolar transistors.